Planning to deliver a baby before 39 weeks can significantly increase the risk of the child having developmental delays, new Australian research has found.

The findings come from a study of more than 150,000 Australian babies born after 32 weeks, who were assessed at kindergarten age.

Doctors looked at the children's physical health and wellbeing, language and cognition, social competence, emotional maturity and general knowledge and communication.

Researchers from the Kolling Institute at Sydney's Royal North Shore Hospital found early "planned" birth before 39 weeks — where mothers were induced or had a planned caesarean — was associated with an increased risk of poor child development at school age.

Study author, professor of obstetrics and gynaecology Jonathan Morris, said almost 10 per cent of the children in the study were found to be developmentally high risk.

"The proportion of developmentally high risk children decreased with increasing gestational age," he said.

The study found:

13.1 per cent of babies born before 37 weeks had poorer developmental scores

10.2 per cent of babies born at 37-38 weeks had worse scores

9 per cent of babies born at 39-40 weeks had poor developmental markers

Obstetricians said the results were significant as early planned birth accounted for almost half of all births in Australia.

"This timing of planned birth is modifiable and the benefits of waiting should be communicated to doctors, parents and families," Professor Morris said.

"The findings support guidelines recommending that being induced or having a pre-labour caesarean for non-medical reasons should not be routinely carried out before 39 and 40 weeks gestation."

Prolonging pregnancy could be good for the brain

Research showed prolonging pregnancy, even at term — 37 to 38 weeks — could benefit brain maturity at birth and potentially improve long-term health and cognitive outcomes.

Health experts said the decision to have a planned birth was predicated on the belief that it is safer for the baby or mother to deliver early.

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"It's the first study to demonstrate increased risk of poorer early childhood development with early planned birth," the study found.

Babies brain development accelerates after 32 weeks gestation with the optimal birth time is 39 to 40 weeks.

Researchers found all modes of birth involving obstetric intervention were consistently associated with an increased risk of developmental delay.

But those babies most at risk of developmental delays were babies those born via caesarean section after being induced.

Why are babies being born earlier?

Experts say many reasons have been given to justify early planned birth, including litigation, patient and doctor perception of safety vs risk, reduced perinatal mortality.

"Despite these justifications and the increasing trend in planned births, national stillbirth and neonatal mortality rates have remained stable and there has been no change in the risk of stillbirth from 36 to 40 weeks gestation," Professor Morris said.

Other studies have shown that developmental problems in childhood can continue into adolescence and adulthood.

Study author Jason Bentley from the University of Sydney's Menzies Centre for Health Policy said there was an urgent need for more informed decision making about planned births.

"In the case where labour occurs naturally before 39 weeks or planned birth is unavoidable, its important there are appropriate interventions and support in early childhood for these potentially vulnerable children," he said.

The findings were published in the international medical journal Paediatrics.